tests/tst-pthread-affinity: Remove bogus struct

tst-pthread-affinity implements a useless copy of pthread.cc's thread_attr
structure. This structure is not used by any code and, hence, can be removed.

Signed-off-by: Renato L. F. Cunha <renato@renatocunha.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>